Simplified finite-element modelling for tunnelling-induced settlements

Tunnelling-induced ground surface settlement prediction still adopts empirical and analytical approaches; thus a step further in using a practical numerical analysis is now a challenging task. Because the deformation during tunnelling is a three-dimensional problem, several features were incorporated in two-dimensional analyses to capture aspects that are important in governing behaviour in the missing third dimension. This paper aims to present simplified methods for ground settlement computation of tunnelling works using the PLAXIS finite-element programme. Three simplified methods – contraction ratio, stress reduction and modified grout pressure – were considered in this study. Practical application requires correlations among these three methods. Such correlations among the three methods are proposed in this study and can be used in geotechnical practice. The results were based on a series of finite-element analyses of the Blue Line Bangkok Mass Rapid Transit tunnels. The geotechnical parameters were selected based on soil investigation reports carried out for construction purposes. The soil constitutive model adopted herein was the hardening soil model on soft and stiff clays. All the finite-element simulations were compared with the measured field deformations. Therefore, the analysis results can be considered as a Class-C prediction (back-analysis)

A hyperplasticity model for clay behaviour: an application to Bangkok clay

The main purpose of this thesis is the development of a new constitutive soil model emphasising the use of thermodynamic principles. This new approach to plasticity modelling, termed ‘hyperplasticity’, was first developed by Collins and Houlsby (1997) and Houlsby and Puzrin (2000). This idea has been further extended to continuous hyperplasticity in which smooth transitions between elastic and plastic behaviour can be modelled (Puzrin and Houlsby, 2001b). Applying hyperplasticity to this research, a kinematic hardening model specified by means of two scalar functionals is used to accommodate the effect of stress history on stiffness. A rate-dependent calculation for an approximation of the incremental stress-strain response is introduced. The model developed in the research is named ‘kinematic hardening modified Cam-clay (KHMCC) model’ and requires eight parameters (plus an extra parameter for rate-dependent analysis). Triaxial test results from the Asian Institute of Technology (AIT) and cyclic undrained triaxial data from Chulalongkorn University are employed to establish the soil parameters for the new model. The model is initially developed in terms of triaxial stress-strain parameters for the purpose of comparison with the experimental data on Bangkok clay. The model is expressed in FORTRAN code for implementation into the OXFEM finite element program. Two examples of real geotechnical projects in Bangkok (a road embankment and tunnelling in soft ground) are analysed under plane strain conditions. Comparisons of the numerical analysis results with field data are made. In addition, factors affecting the results of the analysis such as stress history and K0, are investigated

Predictions of a continuous hyperplasticity model for Bangkok clay

The performance of a new constitutive model called 'kinematic hardening modified Cam clay' (KHMCC) is presented. The model is described using the 'continuous hyperplasticity' framework. Essentially this involves an infinite number of yield surfaces, thus allowing a smooth transition between elasticity and plasticity. The framework allows soil models to be developed in a relatively succinct mathematical form, since the entire constitutive behaviour can be determined through the specification of two scalar potentials. An implementation of the continuous hyperplasticity model is also described. The model requires eight parameters plus a viscosity coefficient for rate-dependent analysis. The model is defined in terms of triaxial stress-strain variables for this study, and is used to model monotonic triaxial tests on Bangkok clay. Comparisons of the theoretical predictions with the results of cyclic undrained triaxial compression tests on Bangkok clay are also presented

Development of hyperplasticity models for soil mechanics

Hyperplasticity theory was developed by Collins and Houlsby (Proc. Roy. Soc. Lon. A 1997; 453: 1975-2001) and Houlsby and Puzrin (Int. J. Plasticity 2000; 16(9):1017-1047). Further research has extended the method to continuous hyperplasticity, in which smooth transitions between elastic and plastic behaviour can be modelled. This paper illustrates a development of a new constitutive model for soils using hyperplasticity theory. The research begins with a simple one-dimensional elasticity model. This is extended in stages to an elasto-plastic model with a continuous internal function. The research aims to develop a soil model, which addresses some of the shortcomings of the modified cam-clay model, specifically the fact that it cannot model small strain stiffness, or the effects of immediate stress history. All expressions used are consistent with critical state soil mechanics terminology. Finally, a numerical implementation of the model using a rate-dependent algorithm is described. Copyright © 2005 John Wiley and Sons, Ltd

Performance of a geosynthetic cementitious composite mat for stabilising sandy slopes

In recent years, there has been much research interest in soil erosion and slope failure due to seepage and rainfall, especially toward finding new technologies/materials with which to stabilise soil slopes. Many geosynthetic materials have been developed to stabilise soil slopes while also being environmentally friendly and convenient for construction. In this study, the performance of a novel geosynthetic cementitious composite mat (GCCM) is studied regarding its ability to stabilise soil slopes. Physical model tests are performed on sandy soil slopes under seepage conditions both with and without GCCM stabilisation. Particle image velocimetry is used to measure the soil displacement, and standpipe piezometers are used to monitor the pore water pressure of the slope. The results show that the slope displacement with GCCM stabilisation is much smaller than that without it. The presence of the GCCM constrains the displacement near the slope surface to being along the slope, whereas without the GCCM the slope can deform freely especially in the middle to upper zone of slope area. The results indicate that the GCCM performs well at slope stabilisation. </jats:p

Model for Shear Response of Asphaltic Concrete at Different Shear Rates and Temperatures

This paper presents a model for shear response of asphaltic concrete, taking into account of strain-rate and temperature effects. The model employs rate-dependent hyperplasticity theory, which is based on a thermomechanical framework. A principle of the theory is that the entire constitutive behavior can be defined by two scalar potentials: an energy potential and a flow potential. The viscous behavior of the model corresponds to the results of rate process theory and defines the strain-rate and time dependent behavior. The initial modulus and shear strength are each assumed to be exponential functions of the inverse of temperature. The model is verified and calibrated against the unconfined compression test data for asphaltic concrete at different strain rates and temperatures. A viscoelastic damage model is also addressed to make a comparison with the model developed here. Comparison between the test data, the predictions of the new model, and the predictions of the viscoelastic damage model are discussed. © 2009 ASCE

Strength and stiffness parameters of Bangkok clays for finite element analysis

Constitutive soil model and its parameters are the important issue in finite element analysis. Hardening soil model and MohrCoulomb model parameters of Bangkok clays for finite element analysis were evaluated in this study. To achieve this purpose, a case study of Sukhumvit MRT Station was selected to model in three dimensions with hardening soil and Mohr-Coulomb models. The instrumented data during construction was used to compare with the results from finite element analysis. PLAXIS 3D software was adopted as solving tool in this study. Lateral wall movement and ground surface settlement predictions were used to compare with the data. The outcomes were concluded that the hardening soil model characterised the Bangkok clay better than Mohr-Coulomb model in 3D finite element analysis for excavation.Full Tex